While the RN is indeed a tiny fraction of what it once was (e,g WW2), Its a bit of a false comparison.
Advances in technology, firepower, range and overall capability (plus massive increase in costs) mean its very difficult compare - and very hard to justify building such a large number of ships anymore.
How many 40's/50's Era destroyers would you trade in for one modern one?
Its come along way from a steamer with a 5" gun and a couple of torpedo launchers.